1. Field of the Invention
The present invention relates to an interferometer and a phase shift amount measuring apparatus which are suitable for EUV (Extremely Ultraviolet) lithography.
2. Description of Related Art
In accordance with the refinement of the semiconductor device, the development of the EUV lithography (EUVL) has been strongly demanded. In the EUVL, an EUV radiation source producing an EUV radiation beam whose wavelength is 13.5 nm is used as the exposure source, and a photomask of reflection type (an EUV mask) is used as the mask. The photomask of reflection type comprises a substrate, a reflection film of a multilayer structure consisting of silicon layers and molybdenum layers formed on the substrate, and absorber patterns formed on the reflection film and functioning as a shielding pattern. In the EUVL, since the exposure beam is projected onto the photomask at the incident angle of 6°, the absorber pattern forms a shadow and thus the degradation of resolution has been pointed out. In order to overcome such problem, it is preferable to thin the thickness of the absorber pattern still more. However, if the thickness of the absorber pattern becomes thinner, its reflectance does not become zero, and thus the absorber pattern may become the shielding pattern similar to the half-tone film which has been used in the phase shift mask of half-tone type. By such a reason, it has been focused on a method in which the absorber pattern for introducing a phase difference of λ/2 between the reflected beams by the absorber pattern and by the surrounding reflection film is provided on the reflection film. If the absorber pattern for introducing the phase difference of λ/2 is formed on the reflection film, the reflected radiations by the absorber pattern and by the reflection film are cancelled each other, and thus the EUVL having high resolution can be established. While, on the other hand, the problem arises that the resolution of the EUVL is degraded if the phase difference caused by the absorber pattern shifts from λ/2. Therefore, in the EUVL, there is urgent need to develop a phase shift amount measuring apparatus which can accurately measure the phase shift amount of the absorber pattern.
As the apparatus for measuring the phase shift amount of the photomask, a measuring apparatus using a Mach-Zender interferometer and a wavefront detecting method has been known (for example, see PLT 1). In this known phase shift amount measuring apparatus, the light source for generating a DUV beam is used as the illumination source, and the illumination beam emitted from the light source is projected onto the rear surface of the photomask through a diffraction grating to form laterally shifted beams. The transmitted beam through the photomask is directed onto the Mach-Zender interferometer. The transmitted beam incident on the Mach-Zender interferometer is divided into two beams by a beam splitter of transflective type. The fringe-scan is performed by a double wedge prism arranged on one optical path to introduce the phase modulation of one period. The two beams are combined by a second beam splitter to form an interference beam. The interference beam is received by a two-dimensional imaging device, and then the phase shift amount is calculated on the basis of the phase shift method using the introduced phase modulation amount and the image signal supplied from the imaging device. In the known phase shift amount measuring apparatus, the phase shift amount is obtained based on the wavefront detection method by using the fringe-scan, and therefore high measurement resolution is obtained without complicate calculation process.
An interference measuring apparatus using two diffraction gratings is also known (for example, see PLT 2). In the known measuring apparatus, incoherent light beam is projected toward the diffraction grating (coherent mask 1) which is arranged at the pupil position of the imaging optical component 2. The diffracted beams of 0-th and +1-th emitted from the grating 1 are projected onto the phase shift mask 4 of transmission type to form an interferogram. The formed interferogram is imaged on another diffraction grating 3 and further is imaged on the detector through the second imaging optical component 5.
PLT1: Japanese Patent Publication (A) No. 2005-83974
PLT2: Published Japanese Translation of PCT international Publication for Patent Application (Kohyo) No. 2009-506335